Summary

This document describes the specification of coordinate systems, and the
interpretation of coordinate values, for spatial region filtering.

Pixel coordinate systems

The default coordinate system for regions is PHYSICAL, which means
that region position and size values are taken from the original
data. (Note that this is a change from the original IRAF/PROS
implementation, in which the IMAGE coordinate system was the default.)
PHYSICAL coordinates always refer to pixel positions on the original
image (using IRAF LTM and LTV keywords). With PHYSICAL coordinates,
if a set of coordinates specifies the position of an object in an
original FITS file, the same coordinates will specify the same object
in any FITS derived from the original. Physical coordinates are
invariant with blocking of FITS files or taking sections of images,
even when a blocked section is written to a new file.

Thus, although a value in pixels refers, by default, to the PHYSICAL
coordinate system, you may specify that position values refer to the
image coordinate system using the global or local
properties commands:

global coordsys image
circle 512 512 100

The global command changes the coordinate system for all
regions that follow, while the local command changes the
coordinate system only for the region immediately following:

local coordsys image
circle 512 512 100
circle 1024 1024 200

This changes the coordinate system only for the region that follows.
In the above example, the second region uses the global coordinate
system (PHYSICAL by default).

World Coordinate Systems

If World Coordinate System information is contained in the data file
being filtered, it also is possible to define regions using a sky
coordinate system. Supported systems include:

If ':' is used as sexagesimal separator, the value is considered to be
specifying hours/minutes/seconds if it is the first argument of a
positional pair, and degrees/minutes/seconds for the second argument
of a pair (except for galactic coordinates, which always use degrees):

At the FK4 1950 coordinates 175.54d RA, 20.01156d DEC exclude a 10
minute by 10 minute box. Then at the FK5 2000 coordinates 179.57d RA
22.4d DEC draw a radial profile regions pattern with 4 quadrants and 5
annuli ranging from 3 minutes to 24 minutes in diameter. In this
example, the default coordinate system is overridden by the commands
in the regions spec.

NB: The Meaning of Pure Numbers Are Context Sensitive

When a "pure number" (i.e. one without a format directive such as 'd'
for 'degrees') is specified as a position or size, its interpretation
depends on the context defined by the 'coordsys' keyword. In general,
the rule is:

All pure numbers have implied units corresponding to the current
coordinate system.

If no coordinate system is explicitly specified, the default system is
implicitly assumed to be PHYSICAL. In practice this means that for
IMAGE and PHYSICAL systems, pure numbers are pixels. Otherwise,
for all systems other than LINEAR, pure numbers are degrees. For
LINEAR systems, pure numbers are in the units of the linear system.
This rule covers both positions and sizes.

As a corollary, when a sky-formatted number is used with the IMAGE
or PHYSICAL coordinate system (which includes the default case of no
coordsys being specified), the formatted number is assumed to be in
the units of the WCS contained in the current file. If no sky WCS is
specified, an error results.

Examples:

circle(512,512,10)
ellipse 202.44382d 47.181656d 0.01d 0.02d

In the absence of a specified coordinate system, the circle uses the
default PHYSICAL units of pixels, while the ellipse explicitly uses degrees,
presumably to go with the WCS in the current file.

Here, the circles use the FK5 units of degrees (except for the
explicit use of pixels in the second radius), while the ellipse
explicitly specifies pixels. The ellipse angle is in degrees.

Note that Chandra data format appears to use "coordsys=physical"
implicitly. Therefore, for most Chandra applications, valid regions
can be generated safely by asking ds9 to save/display regions in
pixels using the PHYSICAL coordsys.